Process for preparing a bleaching liquor containing percarboxylic acid and caro&#39;s acid

ABSTRACT

A process for treating a substrate, e.g., lignocellulosic pulp or cellulosic pulps with a mixed peracid solution comprising percarboxylic acid and Caro&#39;s acid which results in a higher conversion rate of the active oxygen in the hydrogen peroxide in order to provide an inexpensive and effective delignification and/or bleaching solution and the process for making the mixed peracid solution.

RELATED APPLICATIONS

This Application is a Continuation-in-Part of applicants' copendingapplication Ser. No. 07/948,122 filed Sep. 21, 1992, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a process for oxidatively treating asubstrate, e.g., a lignocellulosic material, with a mixed peracidsolution which is an equilibrium mixture of Caro's acid and apercarboxylic acid and to improved methods for preparing such mixedperacid solutions. The present invention also relates to delignifyingand bleaching lignocellulosic materials, and to an improved process forbleaching cellulosic materials, e.g., cotton, with such mixed peracidsolutions.

It is known to use peracetic acid bleaching in the pulp and paperindustry, but such bleaching processes are too expensive and have hadtoo many other shortcomings to be commercially viable. Applicants havediscovered quite unexpectedly and surprisingly that by adding acarboxylic acid, e.g., acetic acid to Caro's acid (H₂ SO₄ +H₂ O₂ ←→H₂SO₅ +H₂ O) the conversion of the hydrogen peroxide reactant, to amixture of peracetic acid and Caro's acid, is substantially increased,without the need for distillation of the peracetic acid, and such mixedperacids solutions have highly desirable pulp delignification andbrightening capabilities.

SUMMARY OF THE INVENTION

In accordance with the present invention, applicants provide a processfor preparing a mixed peracid solution which is an equilibrium mixtureof Caro's acid (H₂ SO₄ +H₂ O₂ ←→H₂ SO₅ +H₂ O) and a percarboxylic acid(RCO₂ H+H₂ O₂ ←→RCO₃ H+H₂ O), wherein R is an alkyl group having 1-5carbon atoms which is useful as an oxidizing agent and/or a bleachingagent. The process comprises mixing the carboxylic acid with Caro's acidwhereby the addition of the carboxylic acid to Caro's acid serves toincrease the conversion of hydrogen peroxide to a mixture of peracids,without the need for a distillation step to remove the percarboxylicacid so formed. In an alternative embodiment of the invention, thesulfuric acid and carboxylic acid components can be premixed and addedto hydrogen peroxide to achieve the same desired high conversion ofperoxide to the mixed peracid solution of the invention.

In accordance with the present invention, applicants also provide animproved process for bleaching cellulosic substrates, such as, forexample, delignified pulp, cotton or cotton by-products with the mixedperacid solution of the invention. These mixed peracid solutions canalso be used in one or more stages of a pulping process to delignifyand/or bleach lignocellulosic pulps. Further, the pulps treated with themixed peracid solutions can previously or subsequently be subjected toother oxidative agents such as oxygen, peroxide, alkali extraction (withor without an oxidant), chlorine, chlorine dioxide and ozone and canalso be treated with a variety of chemical agents and subjected tovarious other process steps.

In practicing the invention, the carboxylic acid, which is preferably a1-5C carboxylic acid, for example formic, acetic, propionic, butyric, orvaleric acid, or a 2-6C dicarboxylic acid, for example oxalic, malonic,succinic, glutaric, or adipic acid, is mixed with Caro's acid in suchrespective amounts that the overall conversion of hydrogen peroxide,which would normally be about 50%, to peracid can be increased to about60-90% while maintaining the mole ratio of carboxylic acid to initialperoxide in the range of from 0.3 to 1.5, but preferably about 0.5-1.2.Alternatively, the added carboxylic acid can be a substituted acid, anaromatic acid or a substituted aromatic acid.

Substrates contemplated for treatment with the mixed peracid solution ofthe invention include lignocellulosic and cellulosic materials, such aswood, wood chips, cotton and cotton by-products, and non-wood plantssuch as grasses, straw bamboo, kenaf, and the like. Lignocellulosicmaterial that have been fiberized in chemical processes, mechanicalprocesses, or combinations thereof such as Kraft, sulfite, soda, NSSC,Organosolv, Thermochemical, Chemithermomechanical pulping areparticularly suitable for treatment by the peracid solution of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating the measurement of the heat of thereaction of acetic acid with Caro's acid over a period of time (20minutes);

FIG. 2 is a graph illustrating the results obtained when an unbleachedkraft pulp was treated with the peracid mixtures of the instantinvention, referred to as Pxa, and peroxymonosulfuric acid (Caro'sacid), referred to as Px, and both samples then subjected to oxygendelignification under the same conditions;

FIG. 3 is a graph illustrating the results obtained when one pulp wastreated with Pxa followed by an oxidative extraction stage reinforced byboth oxygen and hydrogen peroxide and the other pulp was treated withperacetic acid, referred to as Pa, followed by the same oxidativeextraction stage reinforced by both oxygen and hydrogen peroxide; and

FIG. 4 is a graph illustrating the results achieved when a kraft pulpwas treated with two different bleaching sequences utilizing applicants'mixed peracid solution pretreatment and without any use ofchlorine-containing bleaching agents.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is an improved process for preparing a mixedperacid solution which comprises mixing a carboxylic acid of the formulaRCO₂ H, wherein R is an alkyl group of from 1 to 5 carbon atoms withCaro's acid (represented by the equilibrium equation H₂ SO₄ +H₂ O₂ ←→H₂SO₅ +H₂ O) in such respective amounts that the overall conversion of thehydrogen peroxide component (H₂ O₂) can be increased to a value in therange of 60-90% while maintaining a mole ratio of carboxylic acid toinitial hydrogen peroxide of 0.3 to 1.5, but preferably 0.5-1.2. Themole ratio of sulfuric acid to initial hydrogen peroxide is 0.3-3.0,preferably 0.4-1.5. The mixed peracid solution which results has highlydesirable oxidative, pulp delignification and bleaching properties. Themixed peracid solution is an equilibrium mixture of Caro's acid (H₂ SO₄+H₂ O₂ ←→H₂ SO₅ +H₂ O) and the percarboxylic acid component (RCO₂ H+H₂O₂ ←→RCO₃ H+H₂ O).

By adding acetic acid to a Caro's acid mixture produced from a 50 wt.%hydrogen peroxide and a 93 wt.% sulfuric acid solution as shown ingreater detail in Example 3, the overall conversion of peroxide toperacid can be increased from 42% to 80%. In Example 3 the mole ratio ofacetic acid to the initial amount of peroxide is shown as 1.0. Toachieve a similar conversion of peroxide to peracetic acid using 50 wt.%peroxide by previously known equilibrium methods would requireapproximately 6 to 7 moles of acetic acid per mole of hydrogen peroxide.

The results obtained in the instant invention suggest that the aceticacid is partially converted to peracetic acid, and that the sulfuricacid and/or peroxymonoper-sulfuric acids act as the acid catalyst neededfor the reaction. Although the total amount of peracid in the finalmixture can be measured experimentally, the actual distribution of theperacids (H₂ SO₅ and CH₃ CO₃ H) is not known. From the measurement ofthe heat of reaction of acetic acid with the Caro's acid, it appearsthat the reaction is rapid and is complete in about 10 minutes (see FIG.1 of the drawings).

Although the use of 50% hydrogen peroxide and 93% sulfuric acid ispreferred for on-site generation of the peracid mixture, it isrecognized that similar results can be achieved when the strength of thesulfuric acid and/or hydrogen peroxide is lower or higher. The practicalrange for these reagents is 90-99% H₂ SO₄ and 35-70 wt.% H₂ O₂. Theactual choice of reagents would depend on availability of the rawmaterials, but in general the preferred range would be 93-99% H₂ SO₄ and50-70 wt.% H₂ O₂. The concentration of H₂ O₂ can be dictated by therespective amounts of carboxylic acid and H₂ SO₄ utilized in the processof this invention (per 1 mole of H₂ O₂). At certain low ratios ofcarboxylic acid and H₂ SO₄ , respectively, to H₂ O₂, one needs toutilize a sufficiently high concentration of H₂ O₂ to reach the overallconversion of hydrogen peroxide to approximately 60-90%.

Although the process described in the examples is a batch operation, itis recognized by those skilled in the art that the process can also berun continuously. In addition, the order, the amount and the manner ofaddition of the reagents are not critical, and they can be changed asdesired to achieve the objectives of the invention.

It is evident from the disclosure and the examples that the mixedperacid solution obtained from the process of the invention can be usedeconomically and conveniently to treat a variety of substrates,including lignocellulosic and cellulosic materials, such as, forexample, wood, wood chips, delignified pulp, cotton, cotton by-products,and non-wood plants such as grasses, straw, bamboo, kenaf and the like.Lignocellulosic pulps from Kraft, sulfite, soda and other pulpingoperations as well as pulp from recycled paper are especially suitablefor treatment according to the invention. As can be appreciated from theabove and the examples that follow, the process of this inventionutilizes a non-chlorine bleaching agent for the oxidative treatment ofsubstrates in the particular process step which utilizes the mixedperacid solution of this invention.

The invention will now be described through the following examples whichdo not restrict the invention but merely further illustrate the bestmode contemplated by the applicants for the practice of the invention.

EXAMPLE 1 Preparation of Caro's Acid

To a jacketed round-bottom-flask, equipped with a thermometer, agitatorand a dropping funnel was added 72.9 g (1.5 moles) of 70% hydrogenperoxide. Agitation rate was set at 400 RPM and 35° C. water, from aconstant temperature bath, was circulated around the jacket of theflask. The dropping funnel was filled with 225.4 g (2.25 moles) ofsulfuric acid (97.9 wt.% H₂ SO4) and the acid was added dropwise to thecooled mixture over a one hour period while the reactor's temperaturewas kept within a range of 40°-45° C. The mixture was stirred forapproximately 45 minutes and analyzed to determine the conversion ofhydrogen peroxide to Caro's acid (H₂ SO5). The results are shown below:

    ______________________________________                                        H.sub.2 SO.sub.4                                                                        36.15 wt. % 107.83 g   1.10 moles                                   H.sub.2 SO.sub.5                                                                        43.24 wt. % 128.98 g   1.13 moles                                   H.sub.2 O.sub.2                                                                          3.93 wt. %  11.72 g   0.35 moles                                   Active.sup.(a)                                                                           6.06 wt. %  18.08 g   1.13 moles                                   Oxygen                                                                        ______________________________________                                         .sup.(a) Active Oxygen measured as H.sub.2 SO.sub.5                      

Based on these results the calculated H₂ O₂ conversion to H₂ SO₅ was(1.13/1.50)×100 =75%.

EXAMPLE 2 Preparation of Caro's Acid

To a jacketed round-bottom-flask, equipped with a thermometer, agitatorand a dropping funnel was added 102.1 g (1.5 moles) of 50% hydrogenperoxide. Agitation rate was set at 400 RPM and 32° C. water, from aconstant temperature bath, was circulated around the jacket of theflask. The dropping funnel was filled with 238.5 g (2.25 moles) ofsulfuric acid (92.5 wt.% H₂ SO₄) and the acid was added dropwise to thecooled mixture over a one hour period while the reactor's temperaturewas kept within a range of 40°-45° C. The mixture was stirred forapproximately 45 minutes and analyzed to determine the conversion ofhydrogen peroxide to Caro's acid (H₂ SO₅). The results are shown below:

    ______________________________________                                        H.sub.2 SO.sub.4                                                                        47.06 wt. % 160.29 g   1.63 moles                                   H.sub.2 SO.sub.5                                                                        19.90 wt. %  67.78 g   0.59 moles                                   H.sub.2 O.sub.2                                                                          8.79 wt. %  29.94 g   0.88 moles                                   Active.sup.(a)                                                                           2.79 wt. %  9.50 g    0.59 moles                                   Oxygen                                                                        ______________________________________                                         .sup.(a) Active Oxygen measured as H.sub.2 SO.sub.5                      

Based on these results the calculated H₂ O₂ conversion to H₂ SO₅ was(0.59/1.50)×100=40%.

EXAMPLE 3 A. Preparation of Caro's Acid

To a one liter jacketed baffled resin kettle, equipped with athermometer, agitator and a dropping funnel was added 194.4 g (4.0moles) of 70% hydrogen peroxide. The peroxide was then diluted with 97.6g of deionized water (See Note a below). Agitation rate was set at 250RPM and 25° C. water, from a constant temperature bath, was circulatedaround the jacket of the flask. The dropping funnel was filled withconcentrated sulfuric acid (96 wt.% H₂ SO₄) and the initial rate ofaddition of sulfuric acid was set at a rate that allowed the reactionmixture to reach a temperature of about 45° C. The addition rate wasthen reduced and adjusted as needed to maintain the temperature within arange of 40°-45° C. A total of 613 g (6.0 moles) of sulfuric acid wasadded. The mixture was stirred for approximately 30 minutes and analyzedto determine the conversion of hydrogen peroxide to Caro's acid (H₂SO₅). The results are shown below:

    ______________________________________                                        H.sub.2 SO.sub.4                                                                        46.0 wt. %  416.3 g    4.2 moles                                    H.sub.2 SO.sub.5                                                                        21.2 wt. %  191.9 g    1.7 moles                                    H.sub.2 O.sub.2                                                                          8.2 wt. %   74.2 g    2.2 moles                                    Active.sup.(a)                                                                           3.0 wt. %   27.0 g    1.7 moles                                    Oxygen                                                                        ______________________________________                                         .sup.(a) Active Oxygen measured as H.sub.2 SO.sub.5                      

Based on these results the calculated H₂ O₂ conversion to H₂ SO₅ was(1.7/4.0)×100=42%.

B. Preparation of Caro's Acid/Peracetic Acid Mixture

The Caro's acid mixture was cooled to 25° C., and 240 0 g of glacialacetic acid (4.0 moles) was added at once with rapid stirring.Temperature of the mixture rose to 42° C. The mixture was cooled to 35°C. and stored for one hour. At the end of this period a sample of themixture was removed to determine its residual hydrogen peroxide and itsactive oxygen content as peracid (assumed to be a mixture of Caro's acidand peracetic acid). This was done by first analyzing the sample forhydrogen peroxide by the standard ceric sulfate titration method.Potassium iodide was then added to the titrated sample and the activeoxygen content of the sample was measured by titrating the iodinereleased (from the reaction of the iodide with the peracid) with sodiumthiosulfate. The results are shown below:

    ______________________________________                                        H.sub.2 O.sub.2                                                                         1.7 wt. %   19.5 g     0.6 moles                                    Active.sup.(a)                                                                          4.5 wt. %   51.5 g     3.2 moles                                    Oxygen                                                                        ______________________________________                                         .sup.(a) Active Oxygen as mixed peracids                                 

Based on these results the overall conversion of hydrogen peroxide toperacids was 80%. Note a: The procedure described here is equivalent topreparing Caro's acid from 50 wt.% hydrogen peroxide (272.2 g, 4.0moles) and 93 wt.% sulfuric acid (632.8 g, 6.0 moles). For the sake ofexperimental simplicity, the water required (19.8 g) to dilute thesulfuric acid from 96 to 93 wt.% was combined with the water (77.8 g)required to dilute the 70 wt.% peroxide to 50 wt.%. By adding the waterto the peroxide initially, the calculated peroxide strength is 46.7wt.%.

C. Preparation of Caro's Acid/Peracetic Acid Mixture from PremixedSulfuric Acid and Acetic Acid

A premix was prepared by adding 120.0 g (2.0 moles) of glacial aceticacid to 316.3 g (3.0 moles) of 93 wt.% sulfuric acid, (prepared byadding 298.3 g of 98.6% sulfuric acid to 18.0 g of water), and themixture was cooled to room temperature. Using a procedure similar toExample 3A, the cooled premix was added to 136.1 g (2.0 moles) of 50wt.% hydrogen peroxide (prepared by adding 38.9 g of deionized water to97.2 g of 70 wt.% hydrogen peroxide). The reaction temperature wasmaintained at 20°-25° C. during the acid premix addition. The reactionmixture was then stirred for one hour and analyzed, and the results wereas follows:

    ______________________________________                                        H.sub.2 O.sub.2                                                                         2.27 wt. %  13.0 g     0.38 moles                                   Active.sup.(a)                                                                          4.39 wt. %  25.1 g     1.57 moles                                   Oxygen                                                                        ______________________________________                                         .sup.(a) Active Oxygen as mixed peracids                                 

The overall conversion of hydrogen peroxide to the peracids was 79%.

EXAMPLE 4 Effect of Acetic Acid/H₂ O₂ Mole Ratio on Conversion of H₂ O₂to the Peracid Mixture

To a one liter jacketed resin kettle, equipped with a mechanicalstirrer, dropping funnel, and thermometer was added 123.5 g (2.5 moles)of 68.8 wt.% hydrogen peroxide and the mixture was cooled to 15° C.Agitation rate was set at 300 RPM and 372.7 g (3.75 moles) of 98.6 wt.%sulfuric acid was added slowly, using the dropping funnel, whilereaction temperature was kept at 20°-25° C. The reaction mass wasstirred for 45 minutes and analyzed, and the following results wereobtained:

    ______________________________________                                        H.sub.2 SO.sub.4                                                                        36.23 wt. % 179.77 g   1.83 moles                                   H.sub.2 SO.sub.5                                                                        42.33 wt. % 210.04 g   1.84 moles                                   H.sub.2 O.sub.2                                                                          4.29 wt. %  21.29 g   0.63 moles                                   Active     5.94 wt. %  29.47 g   1.84 moles                                   Oxygen                                                                        ______________________________________                                    

Conversion of hydrogen peroxide to H₂ SO₅ =74%,

To a beaker containing 50 g of the above Caro's acid mixture was addedslowly, while keeping the temperature of the mixture at about 10° C.,1.90 g (0.032 moles) of glacial acetic acid. The resulting mixture wasthen analyzed to determine its residual hydrogen peroxide and activeoxygen (as a mixture of peracids) content. The procedure was repeated,each time using an increased amount of acetic acid. The effect ofincreasing the mole ratio of acetic acid to the residual hydrogenperoxide (in the Caro's acid) on conversion of the hydrogen peroxide toperacetic acid is shown below:

    __________________________________________________________________________                                            Conversion(2)                         Acetic Acid                                                                          Sample Weight                    Residual H.sub.2 O.sub.2              Added  (Caro's & Acetic)                                                                      Residual H.sub.2 O.sub.2                                                               Peracid(s)                                                                          Concentration(1)                                                                       to peracid                            (mole) (g)      (Wt. %)                                                                            (mole)                                                                            (% AQ)                                                                              (mole)   %                                     __________________________________________________________________________    0.000  50.0     4.28 0.063                                                                             5.94  0.166    0.0                                   0.032  51.9     1.84 0.028                                                                             5.70  0.165    -1.6                                  0.126  57.6     1.08 0.018                                                                             5.94  0.214    44.4                                  0.188  61.3     0.54 0.010                                                                             6.09  0.233    74.6                                  0.252  65.1     0.14 0.003                                                                             6.01  0.245    93.7                                  __________________________________________________________________________     Notes:                                                                        (1) Moles of peracids expressed as moles of active oxygen                     (2) Conversion calculated as shown in the following example for addition      of 0.126 moles of acetic acid: [(0.214-0.186)/0.063] × 100 = 44.4%      where: Final mole of mixed peracid is 0.214 Initial mole of peracid in        Caro's acid is 0.186 Initial mole of H.sub.2 O.sub.2 in Caro's acid is        0.063                                                                    

The following examples illustrate the use of mixed peracids of theinstant invention for delignifying and bleaching lignocellulosic pulps,and in treatment of cotton or cotton by-products.

EXAMPLE 5 A. Pxa as a Pretreatment Agent for Oxygen Delignification

An unbleached kraft pulp (kappa number 34.5) was treated with Pxa and Pxseparately. In both treatments, 1% equivalent of H₂ O₂ based on pulp wasadded and the pulp samples were treated under the same conditions (70°C., 10% consistency, 1 hour). Both samples were subjected to oxygendelignification under the same conditions. The results are shown in FIG.2. Compared with oxygen delignification without pretreatment or withacid pretreatment in the presence of chelating agent, (Ak)O,pretreatments with Px and Pxa gave much lower kappa number and higherbrightness at a given viscosity as shown in FIG. 2. However,pretreatment with Pxa gave better results than pretreatment with Px.

B. Comparison of Pxa and Caro's Acid (Px)

Southern pine kraft pulp produced by Modified Continuous Cooking (MCC)was oxygen delignified (OD) to yield pulp of kappa number 7.51, 42% ISO,and 17.6 cP viscosity. This pulp was treated in two chelation stages,designated as Q-stages, at 10% consistency with 0.2% (wt/wt OD pulp) ofthe sodium salt of diethylene-triaminepentaacetic acid (DTPA), andwashed with deionized water after each stage. The Q-stages were 40minutes, 70° C. pH 6-7; and 80 minutes, 80° C. pH 4-4. 5, respectively.The resulting pulp was split in equal portions and treated for 4 hourswith 4% (wt/wt OD pulp) hydrogen peroxide, 0.8% DTPA, 0.05% Epsom salts,at an initial pH of 11 8 One portion was held at 90° C., the other at80° C. Brightnesses, L a b values, kappas and viscosities are show incolumn 1 of the following table.

The peroxide treated pulps were also split in equal portions and treatedwith either a Caro's acid solution (Px) prepared as in Example 2, orCaro's acid/peracetic acid solution (Pxa) prepared as in Example 3B. Inboth Px and Pxa treatments, 1% equivalent hydrogen peroxide based onpulp was used. Other reaction conditions were 4 hours, 80 C., 10%consistency, 0 08% DTPA and NaOH sufficient to achieve an initial pH of7-8. Results after washing are also shown in columns 2 and 3 of thetable below.

In both cases the Px treatment resulted in approximately 38-45%delignification but did not significantly improve the pulp brightness,whereas the Pxa treatment gave approximately 60% delignification and 7-8points higher brightness. The Pxa treated pulp also appearedsubstantially less yellow, as shown by the lower b values. Pulpviscosity was not significantly affected by either peracid stage.

    ______________________________________                                        P-stage at 90° C.                                                                1         2       3                                                           QQP       QQPPx   QQPPxa                                            ______________________________________                                        ISO, %      79.2        80.0    86.4                                          L           94.3        94.8    96.1                                          a           -1.33       -1.36   -1.20                                         b           7.52        7.23    4.41                                          kappa       2.74        1.49    1.13                                          viscosity (cP)                                                                            8.0         7.1     7.9                                           ______________________________________                                    

    ______________________________________                                        P-stage at 80° C.                                                                1         2       3                                                           QQP       QQPPx   QQPPxa                                            ______________________________________                                        ISO, %      76.4        76.0    84.3                                          L           94.1        93.5    95.7                                          a           -1.13       -1.69   -1.39                                         b           9.48        8.69    5.56                                          kappa       3.23        2.01    1.37                                          viscosity (cP)                                                                            13.0        12.6    12.3                                          ______________________________________                                    

C. Comparison of Pxa and Peracetic Acid (Pa)

An oxygen delignified kraft pulp produced by a mill in southern USAusing a Modified Continuous Cooking (MCC) process (kappa number 12.9)was used. In one treatment, the pulp was treated with Pxa followed by anoxidative extraction stage reinforced by both oxygen and hydrogenperoxide (Eop). In another treatment, the pulp was treated with Pafollowed by the same oxidative extraction stage. In both Pxa and Patreatments, 1% equivalent of H₂ O₂ based on pulp was used. All otherconditions are identical for both treatments. The results are shown inFIG. 3 of the drawings. The pulp treated with the PxaEop sequence gavehigher brightness and viscosity and lower kappa number than the pulptreated with the PaEop sequence.

EXAMPLE 6 Non-Chlorine Bleaching Sequences Using Pxa

A kraft pulp produced by a mill in Southern USA using MCC process (kappanumber 23.7) with PxaOpPxaOpPxaPP and PxaOpPxaEopPxaPP sequences. Bothsequences gave a final brightness of about 85% ISO without any use ofchlorine-containing bleaching agent as shown in FIG. 4 of the drawings.

EXAMPLE 7 Bleaching of Second-Cut Cotton Linters Using Pxa

Cotton linters were pretreated with 4.5% NaOH solution at 80 psi for 3.5hours and washed at the mill to increase the material's brightness from32 to 47% ISO. The linters were then treated in the laboratory with 1%Pxa (on dry weight of cotton linters; 1% charged is as H₂ O₂ based onthe initial amount of H₂ O₂ used to prepare the Pxa solution) for 60minutes at 100° F. at pH 8 and 6% consistency. The pH was adjusted withdilute NaOH solution prior to heating. The brightness measure afterwashing had improved to 77.7% ISO. A sample treated with 1% H₂ O₂, pH 9,for 60 minutes at 100° F. achieved a brightness of 52.9% ISO. Thebrightness increase for Pxa treated sample, therefore, was about 25% ISOversus the H₂ O₂ treated sample. The Pxa treated sample was furtherbleached to 81.7 ISO brightness with 2% hydrogen peroxide, 1.25% NaOH,for 80 minutes at 140° F. and 12% consistency.

EXAMPLE 8 Bleaching of Second-cut Cotton Linters Using Pxa

Second-cut cotton linters, pretreated with NaOH as above, were bleachedwith Pxa (1% H₂ O₂ equivalent on a dry weight basis) at pH 10 (SampleA), and with 2% Pxa at pH 8 (Sample B) using conditions of Example 7.Brightness of the samples A and B, after the Pxa treatment, followed bywashing, was 75.5 and 75.4 respectively. A sample bleached under thesame conditions using Caro's acid (1% H₂ O₂ equivalent basis) had abrightness of 60.2 ISO. Therefor, brightness increase for Pxa versus Pxwas about 15% ISO. The Caro's acid was prepared following the procedureof Example 2. Sample A and B were subsequently bleached using 2% H₂ O₂under conditions similar to example 7 and attained brightness of 83.2and 82.9 ISO; respectively.

It will be understood that various details of the invention may bechanged without departing from the scope of the invention. Furthermore,the foregoing description is for the purpose of illustration only, andnot for the purpose of limitation--the invention being defined by theclaims.

We claim:
 1. A process for preparing a bleaching liquor containing amixture of percarboxylic acid and Caro's acid which comprises mixing acarboxylic acid of the formula RCO₂ H wherein R is an alkyl group offrom 1 to 5 carbon atoms with an equilibrium solution of Caro's acidcontaining an initial amount of hydrogen peroxide and sulfuric acidwherein the mole ratio of carboxylic acid to the initial amount ofhydrogen peroxide is maintained in the range of from 0.3 to 1.5 and themole ratio of sulfuric acid to the initial amount of hydrogen peroxideis maintained in the range of from 0.4 to 3.0, and wherein theconcentration of H₂ O₂ is sufficient to permit the overall conversion ofH₂ O₂ of approximately 60-90%.
 2. The process of claim 1 wherein thecarboxylic acid is acetic acid.
 3. A process for preparing a bleachingliquor containing a mixture of percarboxylic acid and Caro's acid whichcomprises mixing a carboxylic acid of the formula RCO₂ H, wherein R isan alkyl group of from 1 to 5 carbon atoms with sulfuric acid (H₂ SO₄)and then adding the resulting mixture to a solution of hydrogen peroxideto produce a bleaching liquor containing a mixture of percarboxylic acidand Caro's acid, with the proviso that the mole ratio of carboxylic acidto hydrogen peroxide is in the range of from 0.3 to 1.5 and the moleratio of sulfuric acid to hydrogen peroxide is in the range of from 0.4to 3.0, and wherein the concentration of H₂ O₂ is sufficient to permitthe overall conversion of H₂ O₂ of approximately 60-90%.
 4. The processof claim 3 in which the carboxylic acid is acetic acid.